Technical Abstract:
Olive pomace is the residue produced when olives are pressed for oil. Valuable polyphenolic compounds can be extracted from olive pomace, but this material is more than 60% water (wet basis) and thus costly to transport and process in its original, wet form. The objective of this study was thus to determine the drying behavior of olive pomace under isothermal microwave and microwave-convection drying conditions. ~60g samples of pomace, spread in a flat dish, were dried in a variable-power pilot microwave oven, using a proportional-integral-derivative feedback controller to maintain the center temperature at one of four levels: 40°C, 50°C, 60°C, or 70°C. Each sample was dried either with (4 m/s) or without without (0 m/s) impinging air. Each temperature/fan speed combination was performed in triplicate. Moisture loss was measured gravimetrically every 10 minutes. In order to track the color and geometry of the pomace, a digital photograph of the sample’s surface was taken at each time point. As expected, effective moisture diffusivity increased with increasing temperature in an Arrhenius relationship. However, the effect of impinging air on the effective moisture diffusivity decreased with increasing temperature. The microwave-convection drying curves were best fit to a combination exponential-decay and constant-decay model, indicating the formation of a water barrier crust over time. Examination of the digital photographs confirmed this interpretation. At the highest tested temperature (70°C), the addition of impinging air did not increase effective diffusivity before the crust formed, indicating that microwave heating on its own is sufficient at early times/high moisture content. Thus, a time-dependent approach is recommended for this commodity, with high-temperature microwave (only) heating at early times/high moisture content, followed by high-temperature microwave-convection heating once an oily crust has formed on the product.